1. Field of Industrial Use
The present invention relates to a piston unit used in an internal combustion engine, which causes no interfacial crack between a piston and a wear-proof ring thereof.
2. Prior Art
Referring to FIG. 3, a cylinder 7 of a combustion chamber 70 in an internal combustion engine has a piston unit 9 placed therein, which serves to convert high pressure gas at high temperature generated in the combustion chamber 70 on the cylinder 7 into power. The piston unit 9 is composed of a piston 90 formed of aluminum base alloy, and three piston rings, i.e., a first piston ring 61, a second piston ring 62, and a third piston ring 63, which are inserted into a first ring groove 91, a second ring groove 92, and a third ring groove 93 formed in the piston 90, respectively. Among those piston rings, the first piston ring 61 is inserted into the first ring groove 91 with a wear-proof ring 910 interposed therebetween. The wear-proof ring 910 has been fused with the first ring groove 91.
The aforementioned wear proof-ring 910 serves to protect the first ring groove 91 which is formed in the closest position to the combustion chamber 70 so as to keep it from being worn under severe service condition.
A conventional wear-proof ring 910 has been proposed to employ, for example, nickel base alloy (Japanese Patent Publication No.: 69226 (1992)), cast iron, and the like.
The conventional piston 90 formed of the nickel base alloy is required to be subjected to aluminum coating after fusing of the wear-proof ring 910. The piston 90 formed of the cast iron is likewise required to be subjected to alfin treatment.
Since the wear-proof ring 910 and the piston 90 have different thermal expansion coefficients, interfacial crack might occur therebetween at cooling process after fusing, and further at T6 treatment (quenching, annealing) applied to the piston unit 9. The difference in the thermal expansion coefficients might cause the wear-proof ring 910 to strip off the piston 90, resulting in the piston unit failure.
The piston unit using a ceramics wear-proof ring has been also proposed in Utility Model Application Laid Open No.: 29041 (1986).
In this case, however, there is substantially a large difference in the thermal expansion coefficient between the wear-proof ring and the piston, which fails to allow the aluminum base alloy used for the piston to be subjected to the T6 treatment. As a result, strength and machinability of the piston unit might be degraded.